Electron microscopes such as scanning electron microscopes (SEMS), transmission electron microscopes (TEMs), and scanning transmission electron microscopes (STEMs) require electron lenses which use an electric field or magnetic field to concentrate an electron beam. Lenses acting as spherical lenses that use a rotationally symmetric electromagnetic field are used as electron lenses most widely. Such rotationally symmetric electron lenses are known to inevitably suffer a positive spherical aberration. Even a combination of rotationally symmetric electron lenses cannot make a negative spherical aberration. Accordingly, it is not possible to achieve spherical aberration correction which is achieved by a combination of concave and convex lenses in optics. In traditional electron microscopes, the spherical aberration acts as a main factor that substantially determines the resolution.
On the other hand, it is pointed out that the spherical aberration of an electron lens can be corrected in principle by combing non-rotationally symmetric multipole lenses. However, such a multipole corrector has a complicated structure, since it uses a quadrupole, hexapole, octupole, and the like in multiple stages.
Aberration correction devices include one that generates a hexapole field using a multipole lens to correct the spherical aberration of a rotationally symmetric lens. This aberration correction device performs spherical aberration correction on the following principles. The aberration correction device typically generates a hexapole field using a multiple lens with respect to the positive spherical aberration of an objective lens. Thus, it generates a negative spherical aberration, compensating the spherical aberration of the objective lens. Further, changing the strength of the hexapole field allows the negative spherical aberration to be controlled. Thus, it is possible to control, to any amount, the spherical aberration of the lenses included in the electron microscope, that is, the spherical aberration of the entire optical system, including objective lenses, illumination lenses, and projection lenses. However, the hexapole field generates a second-order aberration. In this case, by disposing two transfer lenses serving as rotationally symmetric lenses between the two multipole lenses and reversing the trajectory of an electron beam between the multipole lenses, the second-order aberration of the hexapole field can be compensated.
Patent Literature 1 discloses an example of a device for correcting the spherical aberration of rotationally symmetric lenses of an electron microscope as described above. FIG. 1 is a schematic diagram thereof. While lenses are illustrated In FIG. 1 as if they were optical lenses, such an illustration is intended for simplification. Actually, these lenses are electron lenses which use a magnetic field.
In an aberration correction device 1, transfer lenses 4 and 5 serving as rotationally symmetric lenses are disposed between multipole lenses 2 and 3, and two transfer lenses serving as rotationally symmetric lenses, 7 and 8, are disposed between the multipole lens 2 and an objective lens 6. The respective focal lengths of the transfer lenses serving as rotationally symmetric lenses are both the same and represented by f; the distance between the transfer lenses 7 and 8 serving as rotationally symmetric lenses by 2f; the respective distances between the multipole lens 2 and the transfer lenses 4 and 8 serving as rotationally symmetric lenses both by f; the distance between the transfer lenses 4 and 5 serving as rotationally symmetric lenses by 2f; and the distance between the multipole lens 3 and the transfer lens 5 serving as a rotationally symmetric lens by f. Traditionally, in making a high-resolution observation using an electron microscope, a specimen position 9 is present in the objective lens 6 serving as a rotationally symmetric lens, and the objective lens 6 serving as a rotationally symmetric lens is used under very strong excitation so that the focal length thereof becomes several mm. An on-axis trajectory 10 is the trajectory of an electron beams passing through the intersection of the specimen and the optical axis and having a certain angle with respect to the optical axis. The on-axis trajectory 10 enters the multipole lens 2 in parallel with the optical axis. Subsequently, the trajectory is reversed by the transfer lenses 4 and 5 serving as rotationally symmetric lenses and enters the multipole lens 3 in parallel with the optical axis as well (spherical aberration correction condition). This spherical aberration correction condition can be met by disposing the multipole lenses 2 and 3 and the transfer lenses 4 and 5 serving as rotationally symmetric lenses as described above. Further, exciting the multipole lenses 2 and 3 to the same degree allows the second-order aberration to be compensated. Specifically, a spherical aberration having a sign opposite to that of the spherical aberration of the objective lens 6 as a rotationally symmetric lens and having an amount corresponding to half the amount of the spherical aberration of the objective lens 6 is provided to the respective hexapole fields of the two multipole lenses, 2 and 3. Thus, it is possible to correct the spherical aberration of the objective lens serving as a rotationally symmetric lens while compensating the second-order aberration.
Further, the aberration correction device in FIG. 1 is configured to correct an on-axis coma aberration. A rotationally symmetric lens has a plane which does not generate an on-axis coma aberration. This plane is called a coma-free plane. Generally, a coma-free plane is located adjacent to the back focal plane of a rotationally symmetric lens. Accordingly, when a high-resolution observation is made by disposing a specimen position 9 in the objective lens 6 serving as a rotationally symmetric lens and using the objective lens serving as a rotationally symmetric lens under strong excitation, a coma-free plane 11 of the objective lens 6 serving as a rotationally symmetric lens is located several mm behind the objective lens 6 serving as a rotationally symmetric lens. Since the distance between the coma-free plane 11 of the objective lens 6 as a rotationally symmetric lens and the transfer lens 7 serving as a rotationally symmetric lens is set to f, the coma-free plane 11 can be transferred to the coma-free plane of the transfer lens 7 serving as a rotationally symmetric lens.
According to the configuration of FIG. 1, the coma-free plane 11 can be transferred to the respective coma-free planes of the transfer lenses 8, 4, and 5 serving as rotationally symmetric lenses on similar principles. If a trajectory passing through the centers of the two multipole lenses, 2 and 3, becomes symmetrical with respect to the midpoint between the multipole lenses 2 and 3, the coma aberration of the multipole lenses can be compensated (coma-free plane transfer condition). In FIG. 1, an off-axis trajectory 12 passing through the coma-free plane 11 of the objective lens 6 passes through the centers of the multipole lenses 2 and 3 and becomes symmetrical with respect to the midpoint between the two multipole lenses, 2 and 3. Thus, the coma-free plane is transferred, correcting the on-axis coma aberration.
As seen, in the aberration correction device having the configuration of FIG. 1, the spherical aberration correction condition is met by the on-axis trajectory 10 between the multipole lenses 2 and 3 forming a rear part of the aberration correction device 1, and the coma-free plane transfer condition is met by the off-axis trajectory 12 between the objective lens 6 and the multipole lens 2 forming a front part of the aberration correction device 1.
Patent Literature 2 discloses the correction of a spherical aberration according to another configuration. FIG. 2 is a schematic diagram thereof. While the configuration between the multipole lenses 2 and 3 forming a rear part of the aberration correction device 1 is the same as that in FIG. 1, the configuration between the objective lens 6 as a rotationally symmetric lens and the multipole lens 2 forming a front part of the aberration correction device 1 differs from that in FIG. 1.
In FIG. 2, the focal lengths of the transfer lenses 7 and 8 serving as rotationally symmetric lenses are represented by f1, f2, respectively; the distance between the coma-free plane 11 of the objective lens 6 as a rotationally symmetric lens and the transfer lens 7 serving as a rotationally symmetric lens by fl; the distance between the transfer lenses 7 and 8 serving as rotationally symmetric lenses by f1+f2; and the distance between the transfer lens 8 serving as a rotationally symmetric lens and the multipole lens 2 by f2. Since the rear part of the aberration correction device 1 has the same configuration as in FIG. 1, the on-axis trajectory 10 meets the spherical aberration correction condition on the same principles. Although the front part of the aberration correction device 1 differs from that in FIG. 1 in configuration, the off-axis trajectory 12 meets the coma-free plane transfer condition, since the transfer lenses 7 and 8 serving as rotationally symmetric lenses are disposed in the positions of the focal lengths.
Features of the aberration correction device in FIG. 2 include ease of fine-adjustment of the spherical aberration. When changing the focal length of the objective lens 6 as a rotationally symmetric lens for focusing or other purposes, the spherical aberration of the objective lens 6 as a rotationally symmetric lens, and the coma-free plane 11 are slightly changed. Accordingly, in the configuration of FIG. 1, the focal lengths f of all the rotationally symmetric lenses must be adjusted. On the other hand, in the fine-adjustment of the spherical aberration correction of the objective lens 6 as a rotationally symmetric lens in the configuration of FIG. 2, the position of the on-axis trajectory 10 passing through the multipole lens 2 is not changed even when fine-adjusting the focal lengths f1, f2 of the transfer lenses 7 and 8 serving as rotationally symmetric lenses. Accordingly, it is possible to correct the spherical aberration without having to change the focal lengths f of the transfer lenses 4 and 5 serving as rotationally symmetric lenses or changing the excitation of the multipole lenses 2 and 3. As a result, according to the configuration of FIG. 2, the following effects can be expected: the spherical aberration correction condition and the coma-free plane transfer condition can be met by fine-adjusting the positions and focal lengths f1, f2 of the transfer lenses 7 and 8 serving as rotationally symmetric lenses, which makes it easy to fine-adjust spherical aberration correction.
FIGS. 1 and 2 show optical systems in TEM observation. An electron beam enters the optical system from the objective lens 6 and exits it from the multipole lens 3 toward a projection lens. The incoming direction of an electron beam in STEM observation is considered to be opposite to that in TEM observation. An electron beam enters the optical system from the multipole lens 3 and exits it from the objective lens 6 toward a projection lens. Since the spherical aberration correction condition and the coma-free plane transfer condition are the same as those in TEM observation, the on-axis trajectory 10 and the off-axis trajectory 12 are the same as those in FIGS. 1 and 2.